I’m not sure how you’d do that for just OOD logins.
But reviewing for a minute, what’s your indication that there’s no systemd user session? From a terminal in the VNC session, does loginctl user-status <username> show anything? (Ensure you specify the username to bypass whatever is set in $DBUS_SESSION_BUS_ADDRESS)
I’m also having an issue with podman over TurboVNC on RHEL 9, and I’m wondering if this is the same thing.
My problem: although there is a systemd user session, processes over the VNC session can’t see it because TurboVNC is setting a unique DBus.